in 1930s. During the Second World War all the roads in Istria sustained significant damage. After the war the road became a part of the Adriatic Highway – a trunk road tracing the Adriatic Sea coast within the SFR Yugoslavia, signposted as M2 route. The next major addition to the road network on the peninsula was completion of the first section of the Istrian Y expressway in 1981. The system was designed to replace M2 route as the main road route in Istria. The Yugoslav M2 designation was retained after the independence of Croatia, until 1997, when the part of the Adriatic Highway spanning the Slovene border and Pula was designated as state road D21. Since October 1991, Hrvatske ceste, a company owned by the Government of Croatia, has been tasked with maintenance of the road. In 2013, following a new classification, the coastal county roads were classified as D75 state road and thus, D21 lost on its importance and was declassified. The present-day route of the D21 consist of short, section of D510, section of Ž5209 county road, forming its northern half and section of D75, forming its southern half. Traffic volume Volume of traffic flowing along D21 road was measured by Hrvatske ceste at six traffic counting stations distributed along the route. Three of the traffic counting stations work continuously, while the rest of them are operated intermittently. By 2002, the road lost a significant portion of its north–south traffic as A9 motorway was gradually extended since 1991, running parallel to the route. At the Kaštel traffic counting site, the average summer traffic volume declined from 12,564 vehicles per day recorded in 2002, to just 2,989 vehicles in 2011. Major intersections The entire route is in Istria County. Sources: Public roads classification and Regulation on motorway markings in Croatia Sources D021 D021